1. Field of the Invention
The present invention relates to a driving circuit for a switch, and more particularly to a floating driving circuit for driving the switch.
2. Description of the Prior Art
A variety of power converters and motor drivers utilize bridge circuits to control power delivery from a power source to a load. The bridge circuit generally has a high-side switch coupled to the power source and a low-side switch coupled to a ground reference. A common node of the high-side switch and the low-side switch is coupled to the load. High-side and low-side switches are generally implemented with transistors. When high-side and low-side switches are controlled to alternately conduct, a voltage level at the common node swings between the power source and the ground reference. Therefore, the voltage level of the common node shifts to the power source when the high-side switch is turned on. In order to fully turn on the high-side switch to achieve low impedance, a gate driving voltage higher than the power source is required. Therefore, gate-to-source of the high-side switch must be floated with respect to the ground reference.
FIG. 1 shows a circuit diagram of a conventional bridge circuit having a bootstrap capacitor 44 and a charge-pump diode 40 to create a floating voltage VCC for driving a gate of a high-side switch 10. The high-side switch 10 receives an input voltage VIN. When a controlling transistor 45 is turned on, the gate of the high-side switch 10 is connected to the ground reference via a diode 42. This will turn off the high-side switch 10. The controlling transistor 45 is controlled by an input signal SIN via an inverter 43. Once the high-side switch 10 is turned off and a low-side switch 20 is turned on, the bootstrap capacitor 44 will be charged up to the floating voltage VCC by a bias voltage VB via the charge-pump diode 40. The low-side switch 20 is coupled to the ground reference. Turning off the controlling transistor 45 will propagate the floating voltage VCC via a transistor 41 to the gate of the high-side switch 10. This turns on the high-side switch 10. A resistor 46 is coupled between the charge-pump diode 40 and the transistor 41.
One drawback of this circuit is its high switching losses in high-voltage applications. The controlling transistor 45 requires a high-voltage manufacturing process to be suitable for high-voltage applications (such as 200 volts or more). However, the parasitic capacitor of a high-voltage transistor is generally large, which will increase a rising time of a switching signal and therefore slow down the switching operation of the high-voltage transistor. This further causes high switching losses of the high-side switch 10. Therefore, this bridge circuit is inadequate for high-voltage and high-speed applications.
Many recently developed bridge circuit designs include methods of generating a suitable gate voltage for the high-side switch. Some well-known inventions include U.S. Pat. No. 5,381,044 (Zisa, Belluso, Paparo), U.S. Pat. No. 5,638,025 (Johnson), and U.S. Pat. No. 5,672,992 (Nadd). These bridge circuits share the same drawbacks as the circuit shown in FIG. 1. The controlling transistors of the aforementioned inventions cause high switching losses in high-voltage applications.
To overcome some of these disadvantages, a bridge circuit utilizing a boost converter technique has been introduced in U.S. Pat. No. 6,344,959 (Milazzo). However, this technique uses a voltage doubling circuit that requires an additional switching element as well as other circuitries, thereby increasing the cost and complexity of the driving circuit. Other prior arts such as U.S. Pat. No. 6,781,422 (Yang) and U.S. Pat. No. 6,836,173 (Yang) disclosed the high-side transistor driver for high-speed applications, but the higher power consumption is still the issue to be concerned.
An objective of the present invention is to overcome the drawbacks of prior arts. Another objective is to eliminate the need of high voltage controlling transistor (such as the controlling transistor 45) for providing a high efficiency driving circuit in high-voltage and high-speed applications.